This invention relates to an improvement in an asymmetric membrane comprising a gel layer and a skin layer formed thereon for use as a reverse osmosis membrane.
A membrane having known Loeb's construction, which has skin layer as an active layer on a face to contact the fed or supplied water and a gel layer for supporting the skin layer on the other side, has been generally used as an efficient reverse osmosis membrane, Ordinarily, such a membrane has been made by, first, applying a suitable solution of high polymer synthetic resin on a porous sheet of fabric or porous plastic film, next, evaporating the solvent to form a skin layer and underlying layer of high polymer resin and then immersing the resultant composite sheet in a gelation liquid thereby to form the Loeb's construction comprising an active skin layer and a supporting gel layer. In such conventional Loeb's construction membrane, the porous sheet remains contact with the gel layer in use. In the conventional semipermeable membrane, a densely woven thick plain-woven cloth, called taffeta, has been generally used to support the Loeb's membrane. In such a membrane, the sheet of the supporting cloth only serves for supporting the membrane.
In the semipermeable membrane, it is an important problem to achieve a higher flux. However, hitherto, it has been difficult to increase the flux without deteriorating rejection against solute of the fed liquid.
The present inventors have found an improved construction of the semipermeable membrane wherein the flux is increased without deteriorating the rejection.